How does Gmail's image proxy work?

Gmail's image proxy intercepts image requests from Gmail users, serving cached copies through Google's infrastructure.

How it works:

1. Gmail receives your email with image URLs

2. When the recipient opens (or Gmail predicts they will), Google's servers fetch all images

3. Images are cached on Google's infrastructure

4. The recipient's browser loads images from Google, not your server

5. Subsequent opens serve from Google's cache

Impact on senders:

IP masking: You see Google's IP addresses, not recipients'. Location tracking from opens becomes impossible for Gmail users.

Cache timing: The first image fetch may not coincide with actual opening. Google may prefetch before the user opens.

Open inflation: Prefetching can trigger false opens (though less aggressively than Apple MPP).

Image updates: Cached images don't refresh immediately. If you change an image at the same URL, Gmail users may see the old version.

Benefits:

Faster loads for recipients (served from Google's fast infrastructure)

Security scanning protects users from malicious images

Reduced bandwidth on your servers

Consistent experience for Gmail's massive user base

Google enabled this by default in 2013. Plan your image strategy knowing Gmail will proxy them.